Fm receiver, noise eliminating apparatus of fm receiver, and noise eliminating method thereof

ABSTRACT

The present invention prevents sound quality from degrading due to a change in electric field intensity or due to multi-path noise. An arithmetic unit subtracts a signal based on the detection signal of multi-path noise whose time constant is specified by a second time constant circuit from a signal based on an RSSI signal whose time constant is specified by a first time constant. An output signal from this arithmetic unit is applied, as a control signal, to a stereo-noise control circuit or the like. This can control the degradation of separation and the like in accordance with multi-path fading to prevent sound quality from degrading.

TECHNICAL FIELD

The present invention relates to a technology for eliminating noise fromreceivers, and more particularly to a technology for eliminatingmulti-path noise from FM receivers.

BACKGROUND ART

A factor for incurring a receiving failure in an FM receiver includesfading in which an FM radio wave propagation route fluctuates when areceiver moves or when radio waves are absorbed or reflected even if thereceiver is fixed and as a result, receiving electric field intensitychange, so-called “multi-path fading”. In this specification, signaldistortion due to this multi-path fading is called “multi-path noise”.

As an FM receiver taking this multi-path noise into consideration, thereis an FM receiver disclosed by Patent Reference 1. In this FM receiver,capacitors C11 and C12 for determining a time constant are provided inparallel. When usually receiving broadcast, this multi-path noise iscoped with by degrading channel separation using a time constantspecified by the capacitor C11. When a receiving mode is switched, timewhen a high frequency signal generated due to receiving mode switchingor the like degrades channel separation is shortened by reducing a timeconstant by switching the capacitor 11 to the capacitor C12 of smallcapacity. Patent Reference 1:

Japanese Patent Application No. H6-140946 (FIG. 1, pp. 2-3)

It is an object of the present invention to provide an FM receiver, anoise eliminator for the FM receiver and a noise elimination method.

DISCLOSURE OF INVENTION

In order to attain the object, the FM receiver of the present inventioncomprises a first detection means, a first time constant setting means,a second detection means, a second time constant setting means, anarithmetic means and a control means.

The first detection means outputs an RSSI signal indicating theintensity of a received radio wave.

The first time constant setting means sets a first time constant in theRSSI signal.

The second detection means outputs a detection signal corresponding to ahigh frequency component due to, for example, multi-path noise includedin an IF signal.

The second time constant setting means sets a second time constant inthe detection signal outputted by the second detection means.

The arithmetic means outputs a signal obtained by subtracting a signalbased on the detection signal from a signal based on the RSSI signal.

The control means controls at least one of a stereo-noise controlcircuit, a high-cut control circuit and a muting circuit.

The present invention can also be realized as a noise eliminator for FMreceivers comprising the first detection means, the first time constantsetting means, the second detection means, the second time constantsetting means and the arithmetic means or a noise elimination methodthereof.

According to the present invention, usually the control signal changebased on the RSSI signal with the first time constant, and for example,when a high frequency component occurs due to multi-path noise, thecontrol signal change based on the detection signal with the second timeconstant. Thus, by using this control signal, a control signal in whichtwo time constants are switched can be realized. In this case, no timedelay also occurs due to this time constant switching.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing the configuration of the multi-pathnoise detection part of the FM receiver of one preferred embodiment ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

One preferred embodiment of the present invention is described belowwith reference to the drawings.

FIG. 1 is a block diagram showing the configuration of the multi-pathnoise detection part of the FM receiver of this preferred embodiment. InFIG. 1, the output of an IF amplifier is inputted as a signal Vin, andmulti-path noise is detected.

The FM receiver of this preferred embodiment has a configuration fordetecting the electric field intensity of a received wave using areceived signal strength indicator (RSSI) and a configuration fordetecting multi-path noise, and a signal obtained by subtracting thedetection signal of the multi-path noise from a signal indicatingelectric field intensity is used as a control signal to a stereo-noisecontrol circuit and the like.

The configuration shown in FIG. 1 comprises a limiter 1, an FM detector(FM DET) 2, a high-pass filter (HPF) 3, an amplitude detector 4, twotime constant circuits 5 and 6, two amplifiers 7 and 8, an arithmeticunit 9, a tuning circuit 10 and resistors R2-R5.

The limiter 1 is provided before the FM detector 2, and eliminates theamplitude fluctuation of signals prior to the FM detection by the FMdetector 2. The limiter 1 shown in FIG. 1 outputs an RSSI signal, whichindicates the change in intensity of an output signal from the IFamplifier as the change of voltage, and has a voltage corresponding tothe intensity of a received wave, in addition to a signal that limitsamplitude.

The FM detector 2 demodulates IF an signal to output an audio signal.The FM detector 2 also outputs a signal for detecting multi-path noise,separately from the audio signal.

The FM detector 2 is a quadrature detector. The FM detector 2 outputsaudio signals by multiplying an IF signal by a signal obtained byshifting the phase of this IF signal by 90 degrees by a phase-shiftedcapacitor C21 and eliminating a component having a frequency signalwhich is the sum of frequency of an FM signal wave and the phase-shiftedsignal from this signal, by the capacitor C22 which acts as low-passfilter. The phase-shifted signal outputted from the capacitor C21 isamplified by the amplifier 22, and high-frequency components of 100 kHzor more are extracted by the high-pass filter 3.

The high-pass filter 3 is a high-pass filter whose pass band is thefrequency of multi-path noise. The high-pass filter 3 extractsmulti-path noise by extracting high frequency components of 100 kHz ormore from the multi-path noise detecting signal outputted from the FMdetector 2.

The amplitude detector 4 outputs high DC voltage according to theamplitude of an input signal.

The time constant circuit 5 sets the time constant of an RSSI signalinputted to the arithmetic unit 9, and comprises a resistor R0 and acapacitor C0. The time constant circuit 6 sets the time constant of themulti-path detecting signal inputted to the arithmetic unit 9, andcomprises a resistor R1 and a capacitor C1. This time constant circuit 6extracts signals of frequencies, for example, 400-500 KHz or more(including an intermediate frequency signal of 10.7 MHz), and thecapacitor C1 of the time constant circuit 6 is charged by voltagecorresponding to the size of multi-path noise. The relationship betweena time constant t0 (=R0·C0) set by the time constant circuit 5 and atime constant t1 (=R1·C1) set by the time constant circuit 6 is t0>t1.

The amplifiers 7 and 8 are provided to balance the impedance between thetwo time constant circuits 5 and 6 provided for two inputs of thearithmetic unit 9, and can be realized, for example, by a voltagefollower using an operational amplifier. Since the resistor R0 of thetime constant circuit 5 is higher than the resistor R1 of the timeconstant circuit 6, the amplifiers 7 and 8 absorb an influence due tothe difference in impedance between the time constant circuit 5 and 6.

The arithmetic unit 9 acts as a subtraction circuit, and outputs asignal obtained by subtracting a signal obtained by amplifyingmulti-path noise inputted via the amplifier 7 by an amplification ratespecified by the resistors R4 and R5 from a signal obtained byamplifying the RSSI signal inputted via the amplifier 8 by anamplification rate specified by the resistors R2 and R3 to thestereo-noise control circuit as a control signal. The stereo-noisecontrol circuit controls channel selection, based on this controlsignal. Since the output of this arithmetic unit 9 contains the carrierfrequency (10.7 MHz) signal component of the intermediate frequencysignal, in order to eliminate this, a low-pass filter must be providedafter the arithmetic unit 9. Alternatively, an OP amplifier with a lowfrequency characteristic must be used for the arithmetic unit 9.

The tuning circuit 10 comprises capacitors C101 and a coil L101, and itsresonant frequency is set to the central frequency (10.7 MHz) of theinput FM signal.

In the configuration shown in FIG. 1, in a normal state where nomulti-path noise is piled on a received wave, the arithmetic unit 9outputs a control signal proportional to the RSSI signal to thestereo-noise control circuit. However, multi-path occurs and multi-pathnoise of a high frequency is piled on the received signal, a signalcorresponding to the amplitude of this high-frequency noise is inputtedto the arithmetic unit 9 via the high-pass filter 3, amplitude detector4, time constant circuit 6 and amplifier 7. The arithmetic unit 9outputs a signal obtained by subtracting a signal obtained by amplifyingthis signal by a specific amplification rate from a signal proportionalto the RSSI signal to the stereo-noise control circuit as a controlsignal. Therefore, usually from the arithmetic unit 9, a control signalwith time constant t0, proportional to the intensity of the graduallychanging intensity of the received wave is outputted. When multi-pathnoise occurs, the level of the control signal decreases for a periodspecified by time constant t1 smaller than time constant t0. Since thelevel of the control signal decreases for a period specified by timeconstant t1 when multi-path noise occurs, in the stereo-noise controlcircuit, separation degrades during the period.

As described above, in the configuration shown in FIG. 1, usually acontrol signal outputted from the arithmetic unit 9 changes, accordingto the RSSI signal with time constant t0, and when a high-frequencycomponent occurs due to multi-path noise, the control signal changesaccording to a detection signal with time constant t1. Therefore, by thestereo-noise control circuit controlling separation using this controlsignal, a control signal in which two time constants are switched can berealized. In this case, no time delay also occurs due to this timeconstant switching. Therefore, the degradation of separation or the likecan be controlled in accordance with multi-path fading, and accordingly,the degradation of sound quality can be prevented.

The control signal outputted from the arithmetic unit shown in FIG. 1can be outputted to a high-cut control circuit (HCC) for attenuating ahigh-band component instead of being outputted to the stereo-noisecontrol circuit to be used only to control multi-separation.Alternatively, the control signal can be outputted to a muting circuitfor performing soft mute (S-mute). Alternatively, this control signalcan control all of the stereo-noise control circuit, high-cut controlcircuit and muting circuit. Alternatively, this control signal cancontrol only one or two of these circuits. Furthermore, all of thesethree or only one or two can also be arbitrarily selected and beswitched and controlled as requested.

Although in the configuration shown in FIG. 1, the output of thephase-shifting capacitor C21 of the FM detector is multi-path noisedetection signal, it can also be configured as shown by dotted lines Binstead of solid lines A in FIG. 1 and audio signal can also be used asthe multi-path noise detection signal. In the case of thisconfiguration, since the audio signal contains signals of a broad-band,the frequency characteristic of the high-pass filter 3 must be set moreseverely than in the case where it is configured as shown by solid linesA. However, if the receiver is provided with a noise canceller, thehigh-pass filter 3 can also be used as a high-pass filter for the noisecanceller.

INDUSTRIAL APPLICABILITY

According to the present invention, separation degradation and the likecan be controlled in accordance with multi-path fading to prevent thedegradation of sound quality. In this case, no time delay due to timeconstant switching also occurs.

1. An FM receiver, comprising: first detection unit outputting an RSSIsignal indicating intensity of a received radio wave; first timeconstant setting unit setting a first time constant in the RSSI signal;second detection unit outputting a detection signal corresponding to ahigh frequency component included in an IF signal; second time constantsetting unit setting a second time constant in the detection signaloutputted by the second detection means; arithmetic unit outputting asignal obtained by subtracting a signal based on the detection signalfrom a signal based on the RSSI signal as a control signal; and controlunit controlling at least one of a stereo-noise control circuit, ahigh-cut control circuit and a muting circuit, according to the controlsignal.
 2. A noise eliminator for an FM receiver, comprising: firstdetection unit outputting an RSSI signal indicating intensity of areceived radio wave; first time constant setting unit setting a firsttime constant in the RSSI signal; second detection unit outputting adetection signal corresponding to a high frequency component included inan IF signal; second time constant setting unit setting a second timeconstant in the detection signal outputted by the second detectionmeans; and arithmetic unit outputting a signal obtained by subtracting asignal based on the detection signal from a signal based on the RSSIsignal as a control signal.
 3. The FM receiver or the noise eliminatorfor the FM receiver according to claim 1, wherein the first timeconstant is larger than the second time constant.
 4. The FM receiver orthe noise eliminator for the FM receiver according to claim 1, whereinthe high frequency component is due to multi-path noise.
 5. A noiseelimination method for a FM receiver, comprising: subtracting a seconddetection signal which has size based on intensity of a high frequencycomponent of an IF signal and has a second time constant from a firstdetection signal which has size proportional to intensity of an IFsignal and has a first time constant, and using a result of thesubtraction as a control signal; and controlling at least one of astereo-noise control circuit, a high-cut control circuit and a mutingcircuit, based on the control signal.
 6. The noise elimination methodaccording to claim 5, wherein the high frequency component is due tomulti-path noise.
 7. An FM receiver comprising: first detection meansfor outputting an RSSI signal indicating intensity of a received radiowave; first time constant setting means for setting a first timeconstant in the RSSI signal; second detection means for outputting adetection signal corresponding to a high frequency component included inan IF signal; second time constant setting means for setting a secondtime constant in the detection signal outputted by the second detectionmeans; arithmetic means for outputting a signal obtained by subtractinga signal based on the detection signal from a signal based on the RSSIsignal as a control signal; and control means for controlling at leastone of a stereo-noise control circuit, a high-cut control circuit and amuting circuit, according to the control signal.
 8. A noise eliminatorfor an FM receiver comprising: first detection means for outputting anRSSI signal indicating intensity of a received radio wave; first timeconstant setting means for setting a first time constant in the RSSIsignal; second detection means for outputting a detection signalcorresponding to a high frequency component included in an IF signal;second time constant setting means for setting a second time constant inthe detection signal outputted by the second detection means; andarithmetic means for outputting a signal obtained by subtracting asignal based on the detection signal from a signal based on the RSSIsignal as a control signal.